Adjust cost_merge_append() to reflect use of binaryheap_replace_first().

Commit 7a2fe9bd improved merge append so that replacement of a tuple
takes log(N) operations, not twice log(N).  Since cost_merge_append knew
about that explicitly, we should adjust it.  This probably makes little
difference in practice, but the obsolete comment is confusing.

Ideally this would have been put in in 9.3 with the underlying behavior
change; but I'm not going to back-patch it, since there's some small chance
of changing a plan choice that somebody's optimized for.

Thomas Munro

Discussion: <CAEepm=0WQBSvuYcMOUj4Ga4NXpu2J=ejZcE=e=eiTjTX-6_gDw@mail.gmail.com>